main sequence

The main sequence of the Hertzsprung-Russell diagram is the curve along which the majority of stars are located. Stars on this band are known as main-sequence stars or dwarf stars. This line is so pronounced because both the spectral type and the luminosity depend only on a star's mass (to zeroth order) as long as it is fusing hydrogen—and that is what almost all stars spend most of their "active" life doing. These main-sequence (and therefore "normal") stars are called dwarf stars not because they are unusually small. They simply have smaller radii and are less luminous than the other main type of stars, the  giant stars. "White dwarfs" are a different kind of star which are indeed small.
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A region on the Hertzsprung-Russell diagram in which stable, middle-aged stars are found. These stars are burning hydrogen in their cores. When the hydrogen ignites, the stars pass through the T Tauri phase and arrive at the zero age main sequence. As the stars age and use up the hydrogen, they change their luminosity and their spectral classification. As a result they move across the main sequence. As the hydrogen burning in its core finally ceases, a star will move right off the main sequence and become a red giant star.

The position, on the main sequence of a Hertzsprung-Russell diagram, reached by stars after they have passed through their T Tauri phase. It corresponds to a star with 75% hydrogen, 23% helium and 2% metals in its core. Nuclear fusion then gradually changes this ratio and the star evolves across the main sequence.